Restoring a corrupt Flash file system
The Flash file system can become corrupt if you lose power while writing to Flash. Although it does attempt to auto-recover corrupt files upon bootup, it is not impossible for corruption to occur anyway. The following is the procedure for reinitializing the Flash file system. This is the equivalent of reformatting your hard drive, except the software remains installed.
First of all, confirm that the Flash file system is corrupt. Open an FTP session and attempt to change your working directory to /FLASH0. If it fails (says "cannot change directory"), this indicates that the directory structure has problems. Being unable to delete an existing file is also a symptom of corrupt file system.
Force a reformatting of the Flash by logging in as the root user, and changing root's password to "killfilesystem" (minus the quotes, and no spaces). Reboot the system. Either a power cycle or soft restart is acceptable. Once the system returns to normal operation, log in as root again, and change the password back to anything but "killfilesystem". If you cannot connect to do this, reboot one more time. The reformatting process takes a couple of minutes, so be patient, but not for longer than 10 minutes.
Note that the above procedure says "reformatting" - this is the same as reformatting the drive in your PC - you will lose any information stored there. If there are files in the Flash file system you wish to save, and they are still accessible, use FTP to retrieve them and store copies outside of the device before reformatting the Flash file system.